1. Field of the Invention
The present invention relates to an electrophotographic printer in which toner is prevented from migrating from a print medium to a heat roller of a fixing unit during fixing operation.
2. Description of the Related Art
FIG. 11 illustrates a relevant portion of a conventional electrophotographic printer.
FIG. 12 illustrates a mechanism in which a toner image formed on a photoconductive drum is transferred onto a print medium.
Referring to FIG. 11, the conventional electrophotographic printer has a charging roller 12, an LED head 13, a developing unit 14, a transfer roller 15, and a cleaning roller 17, all of which being disposed around a photoconductive drum 11. These structural elements are disposed in the order in which a photographic process is carried out.
A print medium P travels in a direction shown by arrow A. The transfer roller 15 is disposed under the photoconductive drum 11 and rotates in contact with the photoconductive drum 11, thereby defining a transfer section through which the print medium passes during printing. When the print medium P passes the transfer section, a toner image is transferred from the photoconductive drum 11 onto the print medium P. The charging roller 12 and cleaning roller 17 are disposed downstream of the transfer section with respect to the rotation of photoconductive drum 11. The LED head 13 is disposed above the photoconductive drum 11. The developing unit 14 is disposed upstream of the transfer section with respect to the rotation of the photoconductive drum.
There is provided an endless belt 10 that is entrained about rollers, not shown, and is sandwiched between the photoconductive drums 11 and transfer rollers 15. The endless belt 10 runs in the direction shown by arrow A. The endless belt 10 attracts the print medium P that is fed from a paper cassette, not shown, and transports the print medium P along a transport path from an upstream end to a downstream end of the transport path.
The charging roller 12 uniformly charges the surface of the photoconductive drum 11. An LED array, not shown, of the LED head 13 is selectively energized in accordance with print data to illuminate the surface of the charged photoconductive drum 11 to form an electrostatic latent image on the photoconductive drum 11.
In the developing unit 14, a sponge roller 14b supplies toner to a developing roller 14a. A developing blade 14c is in contact with the rotating developing roller 14a to form a uniform thin layer of toner on the surface of the developing roller 14a. The developing roller 14a rotates in contact with the photoconductive drum 11, thereby depositing negatively charged toner on the latent image formed on the photoconductive drum 11 to form a toner image.
The transfer roller 15 is connected to a high voltage power supply 16 which applies a potential in the range of several hundred volts to several thousand volts across the photoconductive drum 11 and the transfer roller 15, so that the toner T on the photoconductive drum 11 migrates from the photoconductive drum 11 to the print medium P that is transported by the endless belt 10 between the photoconductive drum 11 and the transfer roller 15.
As shown in FIG. 12, dielectric polarization occurs such that toner image-receiving surface of the print medium P is positively charged. When the negatively charged toner T on the photoconductive drum 11 is brought into contact with the positively charged surface of the print medium P, the negatively charged toner T is attracted by the Coulomb force to the positively charged surface of the print medium P. In other words, the toner image is transferred onto the print medium P. A current flows through the print medium P between the print medium P and non-exposed areas, i.e., areas on the photoconductive drum 11 in which no toner exists, so that the residual charges are neutralized. Thus, the toner image-receiving surface of the print medium P is generally negatively charged. The toner transferred onto the print medium P remains attracted to the print medium by weak Coulomb force.
The cleaning roller 17 shown in FIG. 11 removes the toner that remains on the photoconductive drum 11 after transferring.
A neutralizing brush 18 is provided in the transport path of print medium downstream of the transfer section. The free end of the neutralizing brush 18 is in contact with the non-printed surface of the print medium P in order to neutralize the charges deposited on the non-printed surface of the print medium P. The base portion of the neutralizing brush 18 is grounded.
Downstream of the neutralizing brush 18, there is provided a fixing unit 19 that includes a fixing roller 19a and a pressure roller 19b. The fixing roller 19a has a heat source, not shown, that supplies heat to the surface of the print medium onto which the toner image has been transferred. The pressure roller 19b is disposed under the fixing roller 19a and presses the print medium P against the fixing roller 19a. 
The heat supplied from the fixing roller 19a fuses the toner deposited on the print medium and the pressure applied by the pressure roller 19b causes the fused tone to penetrate the print medium P. The fixing roller 19a and pressure roller 19b have insulating layers formed thereon such that the toner is easily peeled off therefrom.
With the aforementioned conventional electrophotographic printer, the surfaces of the fixing roller 19a and pressure roller 19b are covered with insulation that facilitates the peeling off of toner. However, the conventional printer suffers from the xe2x80x9coffsetxe2x80x9d problem, i.e., an unintentional or faulty transfer of toner from a print medium to the fixing roller in contact with the print medium.
If the offset problem occurs, the toner deposited on the fixing roller 19a is then deposited to the pressure roller 19b and/or other parts of the print medium. The toner may also be deposited on the following page of print medium such that a ghost image may appear on the following page or the page is simply soiled.
The offset problem will occur if the negative potential on the toner image surface of the print medium P and the positive potential on the non-toner image surface are excessively high when the print medium P passes the fixing unit. For a tandem type color printer, the yellow, magenta, cyan, and black toner images are formed on the print medium attracted to the transport belt. Thus, the potential of the toner image surface of the print medium is apt to be higher for a tandem type color printer than for a monochrome printer. Especially, in a low temperature and low humidity environment, the impedance of the print medium is high and therefore the both opposed surfaces of the print medium tend to be high potential.
The surface of the print medium on which a toner image has not been transferred is easily neutralized by the neutralizing brush 18, the frame or chassis of the printer body. However, the charges on the toner-image surface of the print medium and the non-toner image surface are opposite in polarity and are in equilibrium. Thus, it is very difficult to completely neutralize the charges on the both surfaces of the print medium.
If the negative potential on the toner image surface of the print medium and the positive potential on the non-toner image surface are excessively high, when the print medium P is brought into contact with the fixing roller 19a, a potential difference will develop between the fixing roller 19a and the print medium P, creating an electric field therebetween. The electric field causes the negatively charged toner on the toner image surface of the print medium to migrate to the fixing roller 19a, thereby resulting in the offset problem.
An object of the present invention is to provide an electrophotographic printer in which the offset problem of a fixing unit is effectively prevented.
Another object of the present invention is to provide an electrophotographic printer in which the charges deposited on the print medium after transfer are promptly neutralized.
An electrophotographic printer according to the invention incorporates a fixing unit. The fixing unit includes a fixing roller and a pressure roller. The fixing roller has a heat source that generates heat for fusing a toner image transferred by a transfer unit onto a print medium. The pressure roller is in pressure engagement with the fixing roller. At least one of the fixing roller and the pressure roller includes a resilient member that covers a metal shaft and an insulation layer that covers the resilient member. The resilient member contains electrically conductive whiskers that extend radially in three dimensions. The whiskers are dispersed such that a surface of the insulation layer and a surface of the shaft are electrically continuous.
The electrophotographic printer may further include a neutralizing brush that is disposed downstream of the fixing unit in proximity to the fixing unit. The neutralizing brush neutralizes a first surface of the print medium on which the toner image is transferred.
The electrophotographic printer may include a neutralizing roller instead of the neutralizing brush. The neutralizing roller is disposed downstream of the fixing unit and neutralizes a first surface of the print medium on which the toner image is transferred.
The electrophotographic printer may include a metal rod that has a sharp point and is disposed downstream of the transfer unit. When the print medium advances from the transfer unit to the fixing unit, the sharp point is in proximity to a first surface of the print medium to neutralize an excessive charge deposited thereon, the first surface being a surface on which the toner image is transferred.
The electrophotographic printer may further include:
moisture detector that detects a moisture in the air;
moisture calculator that determines an amount of moisture that should be given to the print medium;
a humidifier disposed downstream of the transfer unit and upstream of the fixing unit, the humidifier applying an amount of moisture calculated by the moisture calculator to a second surface of the print medium opposite to a first image on which a toner image is transferred.
Another electrophotographic printer according to the invention incorporates a fixing unit and a neutralizing unit disposed in a transport path of a print medium. The fixing unit includes a fixing roller and a pressure roller. The fixing roller has a heat source that generates heat for fusing a toner image transferred onto a print medium. The pressure roller is in pressure engagement with the fixing roller to form a nip between the fixing roller and the pressure roller. The neutralizing unit is disposed downstream of the fixing unit and neutralizes a charged surface of the print medium discharged from the fixing unit.
The neutralizing unit is disposed to oppose a first surface of the print medium on which the toner image is transferred.
The neutralizing unit is a neutralizing brush.
The neutralizing unit is a neutralizing roller.
Another neutralizing unit may be disposed upstream of the fixing unit and downstream of a transfer section. The another neutralizing unit is disposed to oppose a second surface of the print medium opposite to the first surface.
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.